Radiotherapy is a technique for killing cancerous cells with ionising radiation. The ionising radiation is generated by a radiotherapy device such as a linear accelerator. Such radiotherapy devices typically comprise a multileaf collimator (MLC), which shapes a radiation beam in order to suit a particular patient's clinical need. It is known to attach additional devices, referred to herein as accessories, to radiotherapy devices in order to provide finer control over the shape of the radiation beam. Examples of accessories that are commonly attached to radiotherapy devices include electron applicators, cones, shadow trays and secondary MLCs.
An electron applicator comprises a plurality of planar layers of a radiation-absorbing material, wherein each layer has an aperture through which the radiation beam can pass. In use, an electron applicator is positioned within the radiation beam, such that the beam passes through the aperture of each layer before reaching the patient. Electron applicators provide additional collimation to the beam, and also reduce the patient's exposure to scattered electrons and secondary radiation. Electron applicators having apertures of different sizes and shapes are available, such that an electron applicator having a size and shape that is most suited to a given patient is selected and attached to the radiotherapy device.
A cone is a block of a radiation-absorbing material comprising a bore through which the radiation beam can pass. In use, a cone is positioned within the radiation beam, such that the beam passes through the bore before reaching the patient. The bore further collimates the beam, such that the diameter of the beam when it emerges from the cone corresponds to the diameter of the bore. Cones having bores of different diameters are available, such that the cone that is best suited to a particular patient is selected and attached to the radiotherapy device.
A shadow tray is a tray positioned between the radiotherapy device and a patient. One or more blocks of a radiation-absorbing material, such as lead, are placed on the tray in order to prevent certain portions of the radiation beam from reaching the patient. The radiation beam is thus shaped according to the position of the blocks on the shadow tray.
A secondary MLC is similar to the MLC that is typically already present on a radiotherapy device such as a linear accelerator. The secondary MLC supplements the collimation that is provided by the radiotherapy device's own MLC.
The accessory that is used for a radiotherapy session is chosen by a clinician to suit a particular patient's clinical need. Care must be taken to attach the correct accessory to the radiotherapy device before each radiotherapy session, because an incorrect accessory could result in the patient being exposed to an inappropriately-shaped beam or an inappropriate radiation dose.
To ensure patient safety, the Elekta™ Digital Linear Accelerator comprises an interface that is capable of identifying an accessory that is attached thereto. This interface is described in the “Elekta Digital Linear Accelerator Corrective Maintenance Manual—Accessories” (Elekta Limited, 2012), which is incorporated by reference herein in its entirety. The interface comprises a twenty-four pin Centronics connector. Seven pins of the connector are assigned to carrying a seven-bit code for identifying an electron applicator that is attached to the linear accelerator. Another two pins of the connector are assigned to carrying a two-bit code that allows the linear accelerator to identify whether an electron applicator or a shadow tray is connected thereto. However, the limited number of pins that is available on the connector places an upper limit on the number of different accessories that can be identified via the interface.